1. Field of the Invention
The invention relates to digital-to-analog converters (DACs) and, more particularly, to switched capacitor digital-to-analog converters with power-up/down transient suppression for use in audio systems.
2. Description of Related Art
A number of digital sources of audio information are known. These include compact disk players, digital audio tape, digital transmissions and, similar devices. Stereo digital-to-analog converters are also known, which convert the output from such digital sources into analog information for playback. It is common, when dealing with such stereo digital-to-analog converters, that they have a single-ended output ranging between a ground or return value and a supply voltage level with a nominal or quiescent value, Vq, when no signal is applied. It is common in single-ended output systems to use a D.C. blocking capacitor to provide a ground-centered signal for subsequent processing.
Single-ended circuits powered from a single supply can suffer from large transient signals appearing at the outputs when initially powered on. Such DACs present an analog output centered on a nominal quiescent operating voltage, Vq. The transient occurs when power is applied to the part, and the analog outputs are required to move from a reference level such as ground to Vq. If this transient occurs rapidly, it can be approximated as a step function, which has energy at all frequencies. On power-up, such a system can suffer an annoying xe2x80x9cPOPxe2x80x9d at the speaker as the DAC initially charges the D.C. blocking capacitor to Vq.
A similar click or pop can occur when the system is powered off. On entering the power-down state, the charge on the D.C. blocking capacitor remains. When power is removed, the residual charge on the D.C. blocking capacitor discharges rapidly across the load resister resulting in a loud pop. It would, therefore, be desirable to devise an improved method and apparatus for suppressing such transients in a digital audio device.
In accordance with the invention, electrical circuits provide power-up pop/click transient suppression utilizing a control circuit which operates to replace the pop or click, which would otherwise occur, with a smooth transition.
In accordance with another aspect of the invention, a digital to analog converter suppresses a pop or a click, which would otherwise occur when the DAC is powered down using a current source and a positive feedback amplifier.
Transient signals, and in particular transient signals from DACs using pulse-width modulators that cause audible clicks or pops can be reduced or eliminated by an circuitry that allows the modulator to produce a logic low output signal for some specified period of time during, for example, start up of the device.
Accordingly, one aspect of the present invention provides a transient suppression apparatus. The transient suppression apparatus includes a single-ended AC-coupled output stage, a pulse-width modulated driver and a control circuit. The single-ended AC-coupled output stage drives a load. The pulse-width modulated driver provides output to said single-ended output stage. The control circuit is for setting the output of the driver to produce an output substantially equal to a reference potential found on one side of the load and for gradually adjusting the driver to a nominal operating output thereafter.
Another aspect of the present invention provides a method providing transient suppression. In the method, a pulse-width modulated circuit to produces an output substantially equal to a reference potential found on one side of a load at power up and for gradually adjusting said pulse-width modulated circuit to a nominal operating output thereafter.
Still another aspect of the present invention provides an integrated circuit including a single-ended AC-coupled output stage, a pulse-width modulated driver and a control circuit. The single-ended AC-coupled output stage drives a load. The pulse-width modulated driver provides output to said single-ended output stage. The control circuit is for setting the output of the driver to produce an output substantially equal to a reference potential found on one side of the load and for gradually adjusting the driver to a nominal operating output thereafter.
Yet another aspect of the present invention provides an apparatus including a source of one or more channels of audio information, a digital-to-analog converter, a transient suppression apparatus, and an output system. The digital-to-analog converter converts digital audio information from the source into one or more channels of analog information and includes a pulse-width modulated driver. The transient suppression apparatus has a control circuit for setting the output of the pulse-width modulated driver to produce an output substantially equal to a reference potential for a load and for gradually adjusting the driver to a nominal operating output thereafter. The output system produces audible representations of the one or more channels of audio information.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.